This invention relates to a power steering apparatus mounted on various type of motor cars for decreasing the steering force of the driver.
Recently power steering apparatus have been mounted on various types of motor cars for the purpose of rapidly and smoothly steering the cars, thus decreasing tire of the driver.
The power steering apparatus is constructed to drive an oil pump with such power source as an internal combustion engine or an electric motor and the quantity of oil supplied to the steering device is controlled in accordance with the remaining condition of the motor car so as to smoothly and stably steer the car during a low speed running as well as a high speed running state.
With such prior art power steering apparatus, however, it is necessary to constantly operate the oil pump for the purpose of facilitating steering so that when the power steering device is not used, the power for driving the oil pump results in a power loss. Accordingly, as disclosed in German Laid Open Patent Spcification No. 2,640,988 it has been proposed to drive the oil pump only when the power steering device is operated. This system, however, lacks quick response characteristic where quick steering is necessary to avoid accident. Since small steering force is sufficient during high speed running, a method has been proposed in which the oil pump is stopped when the car speed exceeds a predetermined speed as disclosed in Japanese Patent Publication No. 32145/1981. According to this method power is wasted when the road is crowded with cars or the cars are obliged to frequently stop by signal lamps.